The present invention pertains generally to materials testing and, more particularly, to machinery and methods for applying forces to materials and measuring material responses to applied forces.
Materials testing frequently involves subjecting a material specimen, held in a container or mold, to a variety of forces and analyzing the material response to such forces. In certain types of materials testing equipment, material is held in a rigid cavity or vessel and subjected to substantial force. One inherent limitation of such testing is that by confining a material specimen within a rigid structure such as a mold, it is difficult to detect or measure forces exerted by the material in multiple dimensions or directions. The mold allows measurement of a material response in only a single dimension, which yields data on only certain properties of the material. Measurement of material response in multiple dimensions can yield valuable information on material properties, and in some cases data which corresponds more accurately to the real world characteristics and performance of a material.
In one particular type of materials testing, a composite material may be made up of a mix of a binding/filler material (or xe2x80x9cmasticxe2x80x9d) and an aggregate. Under pressure, the aggregate will shift according to the amount of voids or air pockets in the binding material and according to shear forces between the binding material and aggregate surfaces. Such composite materials are commonly tested by compression within a mold or cavity. Standard testing procedures measure the resistance of the mix to compression only in a single dimension, e.g. resistance to the compressive force. This type of measurement does not account for the lateral forces acting against the cavity walls. A means for measuring the response of a material to an applied force in multiple directions would yield additional useful data on material properties.
One example of a composite material having an aggregate and a fluid binding agent is asphalt mix used for road surfaces, also referred to as hot mix asphalt or xe2x80x9cHMAxe2x80x9d. Known methods for testing the load bearing properties of asphalt involve compaction of an asphalt sample within a mold by a ram driven axially into the mold. Other methods involve movement or gyration of the mold as material is compacted within, as described for example in U.S. Pat. No. 5,456,118, incorporated herein by reference. Asphalt material properties, such as behavior under traffic loads, are deduced from the force applied to the compaction ram, the response or extent of compaction of the mix and from the forces required to gyrate the mold. Because the walls of the mold are rigid, such testing methods do not account for reaction of the material laterally against the mold walls, or in directions other than along the axis of compression. There exists a need to overcome this deficiency of prior art testing methods and equipment.
Identification of tenderness potential (tendency of a material mix to push and shove during in-place compaction) permits rectification through better mix design or modified construction procedure saving a considerable amount of time, energy and money during field rolling of hot mix asphalt (HMA) and cold asphalt mixes. Accurate determination of rutting potential (permanent deformation) of mixes can prevent construction of rut susceptible pavements and associated maintenance and replacement costs. Numerous studies have shown that the rutting potential of HMA increases significantly with an increase in asphalt content and an increase in the percentage of rounded aggregates. Studies have also shown that rutting potential of HMA increases significantly as the air voids drop below two percent. Experience from all over the world also indicates that stone matrix asphalt (SMA) has significantly less rutting potential compared to dense graded HMA, even at low air voids. To date many theories and equipment have been developed to simulate the phenomenon of rutting in the laboratory and hence to predict the rutting potential of mixes. However, no theory or equipment has so far been completely successful in predicting rutting potential of asphalt mixes in a way which matches the performance of in-place mixes.
At present there is a need for a single tool that can predict the tenderness and rutting potential of asphalt paving mixes accurately. Furthermore, there is a need for a tool that can accurately identify the rutting potential of mixes at different air voids (voids in mix, VTM).
It is therefore an object of the present invention to provide a tool that can be used to: determine rutting potential of mixes; to determine tenderness potential of mixes; to determine ideal design air voids for different mixes; to compare different asphalt binders; to compare different aggregates; to compare different gradations; and to control quality of mixes during production and laydown.
The invention provides in one aspect a newly developed tool, a lateral pressure indicator (LPI), for predicting tenderness and rutting potential of composite or mixed materials, such as asphalt paving mixes. The present invention provides a material testing method and system in which the response of a material to an applied force is measured in a direction other than that of a force applied to the material. In accordance with one general aspect of the invention, there is provided a system and method for measurement of reaction of a material in multiple dimensions in response to an applied force, and methods for determining material properties from the measured reactions. In one example of the invention, lateral pressure of a mix material generated as a result of a vertical or non-aligned applied pressure or force is measured. Materials which exert a high lateral pressure relative to an applied vertical pressure are identified as having an accompanying low shear strength. In the case of an asphalt/aggregate mix proposed for use as a paving surface, the measured lateral pressure and shear strength properties are predictive of the performance of the asphalt mix in a real traffic environment.
In one embodiment of a material testing system of the invention, a device is provided for measuring reactive forces of a material subjected to a testing force. The device comprises a mold capable of housing material to be tested adapted for use with material testing equipment. The mold has a mold wall, a portion of which has an aperture defined by a perimeter within the mold wall area. The aperture in the mold wall is configured to accept an insert piece. A sensor is provided for measuring the force exerted on an insert piece by material in the mold.
In one specific embodiment of a material testing mold of the invention, for use in connection with material testing equipment operative to apply a force to material within the mold and to gyrate the mold as the force is applied, the mold has a mold cavity defined by a mold wall and an opening through which a compaction ram enters the mold to compact material within the mold, a portion of the mold wall being movable in a direction generally orthogonal to a direction of compaction of material within the mold by the material testing equipment. One or more sensors such as load cells are operatively connected to the movable portion of the mold wall to sense a force exerted on the movable portion of the mold wall by the material compacted in the mold.
In another specific embodiment of a material testing mold of the invention, for use in connection with material testing equipment, the mold has a mold cavity defined by a continuous mold wall. A portion of the mold wall is configured to deflect or otherwise dynamically respond to a force exerted on the wall by material within the mold which is under a compaction force applied by the material testing equipment to the material within the mold. The portion of the mold wall configured to deflect or dynamically respond to pressure of material under compaction within the mold is instrumented to measure a force exerted by the material upon the wall. The measured force is indicative of physical properties and load bearing characteristics of the material in the mold, including shear strength.